The present invention relates generally to systems that employ hybrid ARQ schemes and in particular, to a reliability-based type-II hybrid ARQ scheme.
Digital data transmissions over wired and wireless links sometimes may be corrupted, for instance, by noise in the link or channel, by interference from other transmissions, or by other environmental factors. Even with clear communication channels, which lend themselves to high data rates, it may not be possible to appropriately decode the data stream with the requisite error rates. In order to solve this problem, many current communication systems employ an automatic repeat request (ARQ) scheme for retransmission. In such systems an opportunity exists for requesting that data be retransmitted upon detection of an error. In more complex systems a type-II hybrid ARQ scheme is employed. In systems employing a type-II hybrid ARQ (HARQ) scheme a receiver combines previously received erroneous transmissions of a packet of information with a newly received transmission in an effort to successfully ascertain the true contents of the packet. In other words, coded bits received on a first erroneous transmission are stored and combined with the coded bits received on later transmissions thereby increasing the likelihood of a correct decoding on later transmissions. Similarly the coded bits received on the second or later transmissions are stored for combining with subsequent received bits.
While the above-described HARQ scheme does eliminate many errors that occur during transmission, no attempt is made by the system to identify the size or content of the information needed at the receiver to execute a successful decoding, and to request that information from the transmitter. Because of this, it is very likely that retransmissions include information that is unnecessary, decreasing overall system capacity. Therefore, a need exists for a method and apparatus for a HARQ scheme that minimizes the transmission of unnecessary information.